Hmmm... not to sound too flippant, perhaps it controls the brakes
I think you need to be a bit more specific. Are we talking about cars, electric motors, elevators, or what? You may also want to explain how much detail you're looking for. There's a lot of good advice on this forum, but you need to state your question more clearly, and please check back often - at least once a day - and provide feedback to the responses. That encourages us to be even more helpful
I'd say that answer is equal to the question. Both are simple and to the point.
Also, the question is typical of so many open ended ones on all the forums. It seems someone hears a statement and immediately thinks it must mean something deep not simply "that thing you press on with your foot when you want to stop your vehicle."
It enables the driver to apply a varying amount of mechanical resistance to the rotating wheelsets and thereby bring the train to a graduated, controlled stop before either SPADding [SPAD is an acronym for "Signal passed at danger"], overshooting the platform, or hitting the buffer stops.
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Good call! That's another definition of "brake" that I hadn't even considered. Proves my point though about having the Question Poser provide more details.
So how about it QP? Are you following this or have you abandoned this thread like so many in other threads ?